Ion implantation systems are used to impart impurities, known as dopant elements, into semiconductor substrates or wafers, commonly referred to as workpieces. In such systems, an ion source ionizes a desired dopant element, and the ionized impurity is extracted from the ion source as a beam of ions. The ion beam is directed (e.g., swept) across respective workpieces to implant ionized dopants within the workpieces. The dopant ions alter the composition of the workpieces causing them to possess desired electrical characteristics, such as may be useful for fashioning particular semiconductor devices, such as transistors, upon the substrates.
The continuing trend toward smaller electronic devices has presented an incentive to “pack” a greater number of smaller, more powerful and more energy efficient semiconductor devices onto individual wafers. This necessitates careful control over semiconductor fabrication processes, including ion implantation and more particularly the uniformity of ions implanted into the wafers. Moreover, semiconductor devices are being fabricated upon larger workpieces to increase product yield. For example, wafers having a diameter of 300 mm or more are being utilized so that more devices can be produced on a single wafer. Such wafers are expensive and, thus, make it desirable to mitigate waste, such as having to scrap an entire wafer due to non-uniform ion implantation. Larger wafers and high density features make uniform ion implantation challenging, however, since ion beams have to be scanned across larger angles and distances to reach the perimeters of the wafers, yet not miss implanting any region there between.
In addition, high voltages supplied to the ion source for such an ion beam are subject to occasional arcing between the various high voltage electrodes and other nearby parts. This tendency for arcing often fully discharges one or more affected high voltage (HV) power supplies until the arc naturally self-extinguishes at a much lower supply voltage. While arcing, the beam current may become erratic or may be interrupted until the supply voltage is restored, during which time ion implantation may experience intermittent or non-uniform dose levels across the workpiece.
Arcing can occur if a film forms on a surface during the course of processing/implanting wafers, whereby the film becomes delaminated and falls in a high voltage gap between two electrodes. The film may also become charged and embedded in the ion beam until it is transported downstream across a high voltage gap that precipitates an arc. The arcing may ablate the film material, thus generating a large amount of particles which may also become embedded in the wafer. Arcing can also occur after insulators and/or feedthroughs become coated with process material or byproducts to the point that their insulation values become insufficient to isolate the HV, resulting in an arc that may track across the insulator/feedthrough and ablate material, restoring some amount of insulation value repeatedly until the HV power supply can be maintained, or the implantation system is taken out of service. Arcing can also occur due to vacuum leaks and/or pressure bursts near high voltage stress fields.
Arcs may form between at least one high voltage electrode and another conductive component. Three different types of arcing are illustrated in a conventional ion implantation system 10 shown in FIG. 1. A first arc type 12 occurs between an ion source electrode 14 (which is at a positive potential) and an extraction ground electrode 16. A second arc type 18 occurs between a suppression electrode 20 (which is at a negative potential) and the ground electrode 16 or other grounded electrode that is proximate the suppression electrode. The first arc type 12 can sometimes induce or cascade into an additional arc of the second arc type 18. These first and second arc types 12, 18 may be caused by mistuning of the ion implantation system 10 due to a software or operator error. A third arc type 22 occurs between an electrode (e.g., the ion source electrode 14) and a housing 24 that is proximate the electrode. Other types of arcing include arcing between two electrodes of the same polarity, such as between a terminal bias electrode and a terminal suppression electrode. Arcing to the beamline surrounding any electrode at potential is often present as well.
FIG. 1 further illustrates a high positive voltage extraction supply 26 that feeds extraction slits of the source electrode 14, and a high negative voltage suppression supply 28 that feeds the suppression electrodes 20 neighboring the ground electrodes 16. The HV suppression supply 28 has a conventional arc suppression or protection circuit 30, which may use a current limiting resistor 32 to limit the arc current to the suppression electrodes 20, a capacitor 34 to filter and stabilize the voltage of the supply, and a fly-back diode 36 to limit any reverse voltages generated from reactive elements of the circuit during arc on-off cycling.
Conventionally, the arc protection circuit 30 limits the arc current based on a fixed threshold current. Use of a fixed threshold current, however, can limit the effectiveness of an arc protection circuit 30 because the threshold should be set high enough to avoid false triggering. However, due to different process recipes and operating conditions, the current being supplied by the various power supplies may vary enough to make a fixed threshold current ineffective for detecting some arcing conditions in a timely manner, if at all. Accordingly, there is a need for detecting an arc under various circumstances to allow for mitigation of the effects of high voltage arcing associated with an ion source or various electrodes of an ion implantation system.